Thermal imaging methods and materials

ABSTRACT

Leuco dyes are provided which comprise the coupling product of a N-acyl substituted aromatic amino color developer and a dye-forming coupler moiety substituted at the coupling carbon with a thermally removable leaving group. Thermal imaging systems employing these leuco dyes have the advantage of reduced bubble formation relative to thermal imaging systems employing prior art leuco dyes containing a group which thermally fragments into one or more gases.

REFERENCES TO RELATED APPLICATIONS

Copending application Ser. No. 07/548,223 filed Jun. 29, 1990 by L. D.Taylor and D. P. Waller and assigned to the same assignee as the presentapplication, describes and claims leuco dyes which generate a dye uponapplication of heat.

Copending application Ser. No. 07/277,014 filed Nov. 28, 1988 by RogerA. Boggs et al, (now abandoned and replaced by copending U.S. patentapplications Ser. Nos. 07/729,420 (now U.S. Pat. No. 5,192,645) and07/729,426, bow filed on Jul. 12, 1991), and assigned to the sameassignee as the present application, describes and claims leuco dyeswhich generate a dye upon the application of heat.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to heat-sensitive recording elements particularlyuseful for making color hard copy, to a method of imaging employing saidelements and to novel leuco dyes (the term "leuco dye" is used herein torefer to a substantially colorless compound which generates a coloredmaterial upon heating) and the dyes derived therefrom useful as thecolor image-forming materials.

2. Description of the Relevant Art

Leuco dyes have been suggested which become irreversibly colored by theloss of a single group. For example, Japanese Patent Kokai No. 57-46239,Laid Open Mar. 16, 1982, discloses colorless indoaniline compounds whichpossess an alkyl/aryl sulfonyl group that irreversibly cleaves from themolecule upon exposure to light, usually ultraviolet light, with theresult that the compound is converted to its colored form. U.S. Pat. No.3,409,457 to Karl-Heinz Menzel discloses leuco dyes which possess anacylamino group that cleaves from the molecule upon heating to yield acolored azomethine dye. The conversion of these leuco compounds into theazomethine dyes is accelerated by using alkalis such as alkalialcoholates. The acylamino and alkyl/aryl sulfonyl groups employed inthe compounds of these references depart from the molecule to effectconjugation and form a dye chromophore.

U.S. Pat. No. 4,720,449 to Alan L. Borror and Ernest W. Ellis disclosescolorless di- and tri-arylmethane compounds possessing a maskedacylation substituent which undergoes irreversible fragmentation uponheating to liberate the acyl group for effecting an intramolecularacylation reaction whereby the compounds are rendered colored.

The copending application Ser. No. 07/548,223 of L. D. Taylor and D. P.Waller describes and claims leuco dyes comprising mixed carbonate estersof quinophthalone dyes and tertiary alkanols containing not more thanabout 9 carbon atoms. Application of heat to the leuco dyes causes thebreakdown of at least one carbonate ester grouping in the mixed ester,whereby the compounds are rendered colored. The preferred esters are thetertiary-butoxycarbonyl (hereinafter t-Boc) derivatives.

U.S. Pat. No. 4,602,263 to Alan L. Borror, Ernest W. Ellis and Donald A.McGowan discloses the stabilization of a leuco dye by employing atertiary-alkoxycarbonyl group, for example, t-Boc as a thermallyremovable protecting group. This protecting group is removed byunimolecular fragmentation upon heating, which fragmentation reaction isirreversible. Copending U.S. Patent Application of Roger Boggs, et al(Ser. No. 07/277,014, now abandoned and replaced by copending U.S.patent application Ser. Nos. 07/729,420 (now U.S. Pat. No. 5,192,645)and 07/729,426, both filed on Jul. 12, 1991), discloses leuco dyes whichupon the application of heat become irreversibly colored by the loss ofa leaving group and a thermally removable protecting group. Both theleaving group and the thermally removable protecting group are requiredto stabilize the colorless leuco dyes until the application of heat.

The thermally removable protecting groups employed in the aboveidentified U.S. Pat. No. 4,602,263 and copending U.S. Patent Applicationof Roger Boggs, et al (Ser. No. 07/277,014, now abandoned and replacedby copending U.S. patent application Ser. Nos. 07/729,420 (now U.S. Pat.No. 5,192,645) and 07/729,426, both filed on Jul. 12, 1991) and theesters in the above-identified copending application, Ser. No.07/548,223, undergo fragmentation. This fragmentation involves theformation of one or more gases (i.e., compounds which exist as gases atroom temperature and atmospheric pressure), e.g., when t-Boc is employedas the thermally removable protecting group or as the preferred ester itundergoes thermal fragmentation to liberate two gases, i.e., carbondioxide and isobutylene. These gases become trapped within the imagingsystem in the form of bubbles. Bubbles are undesirable because theycause light scattering and consequently impair image quality byproducing areas which appear dark in transmitted light.

The present invention provides leuco dyes and methods and materials forthermal imaging which have the significant advantage of reduced bubbleformation.

SUMMARY OF THE INVENTION

This invention provides leuco dyes which comprise the coupling productsof a N-acyl substituted aromatic amino color developer and a dye-formingcoupler moiety substituted at the coupling carbon with a thermallyremovable leaving group. These leuco dyes may be represented by ##STR1##wherein: E represents a thermally removable leaving group;

tM represents a thermally migratable acyl group;

Q, Q' and C taken together represent a dye-forming coupler moietywherein C is the coupling carbon of said coupler moiety; and,

(Y) taken together with N represents an aromatic amino color developermoiety,

one of said Q, Q' and (Y) containing an atom selected from the atomscomprising Group 5A/Group 6A of the Periodic Table, said groups E and tMmaintaining said leuco dye in a substantially colorless form until theapplication of heat causes said group E to be eliminated from said leucodye and said group tM to migrate from said N atom to said Group 5A/Group6A atom thereby forming a dye represented by ##STR2## wherein saiddotted lines indicate that said tM group is bonded to said Group5A/Group 6A atom in one of said Q, Q' and (Y).

This invention also provides a heat-sensitive recording element whichcomprises a support carrying at least one layer of the aforementionedleuco dyes.

This invention additionally provides a method of thermal imaging whichcomprises heating imagewise a heat-sensitive element comprising asupport carrying at least one layer of the aforementioned leuco dyes,thereby causing, in the heated areas, the elimination of the thermallyremovable leaving group and the thermal migration of the acyl group,whereby the leuco dye is converted into a colored dye of this inventionin an imagewise pattern corresponding to the imagewise heating.

This invention also provides the aforementioned dyes.

DETAILED DESCRIPTION OF THE INVENTION

The leuco dyes of the present invention, as mentioned above, comprisethe coupling products of an N-acyl substituted aromatic amino colordeveloper and a dye-forming coupler moiety substituted at the couplingcarbon with a thermally removable leaving group. The leuco dyes may berepresented by: ##STR3## wherein: E represents a thermally removableleaving group;

tM represents a thermally migratable acyl group;

Q, Q' and C taken together represent a dye-forming coupler moietywherein C is the coupling carbon of said coupler moiety;

and,

(Y) taken together with N represents an aromatic amino color developermoiety,

one of said Q, Q' and (Y) containing an atom selected from the atomscomprising Group 5A/Group 6A of the Periodic Table, said groups E and tMmaintaining said leuco dye in a substantially colorless form until theapplication of heat causes said group E to be eliminated from said leucodye and said group tM to migrate from said N atom to said Group 5A/Group6A atom thereby forming a dye represented by ##STR4## wherein saiddotted lines indicate that said tM group is bonded to said Group5A/Group 6A atom in one of said Q, Q' and (Y). Group 5A/Group 6A atomsmay be, for example, nitrogen, oxygen and sulfur, the preferred atombeing nitrogen.

As described above, the thermally migratable acyl group (tM), upon theapplication of heat, migrates from its original nitrogen atom to a Group5A/Group 6A atom contained within one of said group Q, group Q' and saidgroup (Y) of the leuco dye. The tM group may be represented by thestructural formula ##STR5## wherein R is alkyl usually having 1 to 12carbon atoms such as methyl, isopropyl, and hexyl; cycloalkyl such ascyclopentyl and cyclohexyl; aryl such as phenyl; aralkyl such as benzyland phenethyl; alkaryl such as methylphenyl and ethylphenyl;heterocyclic such as pyridine, quinolidine, pyran, thiophene and furan;and, ##STR6## wherein R' and R", the same or different, are selectedfrom hydrogen; alkyl usually having 1 to 12 carbon atoms such as methyl,isopropyl, and hexyl; cycloalkyl such as cyclopentyl and cyclohexyl;aryl such as phenyl; aralkyl such as benzyl and phenethyl; alkaryl suchas methylphenyl and ethylphenyl; heterocyclic such as pyridine,quinolidine, pyran, thiophene and furan.

The thermally removable leaving group, E, may be any leaving group thatis nucleofugal, i.e., a leaving group which departs with the bondingelectron pair. Such leaving groups must necessarily be capable ofstabilizing the electron pair once the leaving group has beeneliminated. Thus, it will be clear to one skilled in the art thathydrogen is not a nucleofugal leaving group. Nucleofugal leaving groupsare well known and have been discussed by Charles J. M. Stirling, Acc.Chem. Res., 12, 198 (1979) and by Charles J. M. Stirling, et al., J.Chem. Commun., 940 (1975). Examples of leaving groups that can beemployed in the present invention include heterocycles such asimidazolyl or ##STR7## halo such as chloro; hydroxy; SOR'"; SOAr;--SR'"; --SO₂ R'"; --SAr; --SO₂ Ar; --SeAr; --OAr; --OR'"; P(O)(OR'")₂ ;--C(R'")₂ EW; --C(R'")(EW)₂ ; --CH(EW)₂ ; --N(R'")Ar; --N(Ar)Ar;--N(Ar)CO2CH₂ Ar; and --N(R'")CO₂ Ar wherein EW represents anelectron-withdrawing group, R'" is alkyl and Ar is aryl, usually phenyl,unsubstituted or substituted with one or more substituents, e.g., alkyl,alkoxy, halo, carboxy, nitro, cyano, --SO₂ alkyl, --SO₂ phenyl, tosyland N,N-(dialkyl)amino wherein said alkyl usually contain 1 to 6 carbonatoms. As used herein and as is well known in the art, an electronwithdrawing group is a group having a positive sigma value according toHammett's equation. The preferred leaving groups are phenoxy,unsubstituted or substituted with one or more groups, for example, alkylusually having 1 to 20 carbon atoms, and carboalkoxy usually having 1 to20 carbon atoms.

As described by Nassau, K. in The Physics and Chemistry of Color, JohnWiley and Sons, New York. 1983. p.110, a dye is defined as a"color-producing chromogen, which is composed of a basic chromophore("colorbearing") group, not necessarily producing color, to which isattached a variety of subsidiary groups, named auxochromes ("colorincreasers"), which lead to the production of color. Chromophoresinclude carbon-carbon double bonds, particularly in conjugated systemscontaining alternating single and double bonds as in the carbon chain,structure (6-1), as well as in the azo group, structure (6-2), thiogroup, structure (6-3), and nitroso group, structure (6-4), amongothers. ##STR8## Auxochromes include groups such as --NH₂, --NR₂ where Rrepresents an organic group, --NO₂, --CH₃, --OH, --OR, --Br, --Cl, andso on. We now recognize that some of these auxochromes are electrondonors, such as --NH₂, and some are electron acceptors, such as --NO₂ or--Br." For a further discussion of the auxochromophoric system of dyes,see Gilman, H., Organic Chemistry, An Advanced Treatise, Vol. 111, JohnWiley & Sons, N.Y., 1953, pp. 247-55; and Venkataraman, K., TheChemistry of Synthetic Dyes, Vol. I, Academic Press, Inc., New York,1952, pp. 323-400. The term "color" is used herein to mean "absorbingelectromagnetic radiation of a particular wavelength" and does notnecessarily refer to visible radiation. Thus, a colored dye of thisinvention includes a material which absorbs infrared or ultravioletradiation.

The thermally removable leaving group (E) and the thermally migratableacyl group (tM) are positioned on the leuco dyes of the presentinvention in the manner described above so as to interrupt theconjugation of the colored auxochromophoric system and render itsubstantially colorless. Said tM and said E are required to stabilizethe colorless form until the application of heat causes the conversioninto dyes. This conversion from the colorless to the colored form isachieved by the thermally induced elimination of group E and themigration of group tM from its original position on the nitrogen atom ofthe developer moiety to some other Group 5A/Group 6A atom containedwithin the leuco dye, thereby effecting conjugation in the chromophoreposition to generate a dye and hence color formation.

To avoid premature coloration under normal storage and handlingconditions, E and tM are selected such that said Group E is eliminatedand group TM migrates substantially from the leuco dye only at anelevated temperature.

It is well known in the photographic art that color developers areoxidized and react with dye-forming couplers to form a wide variety ofcolors. The dye-forming coupler moieties, of the present invention, inthe above formulae, represented by ##STR9## may be monomeric orpolymeric and may be any of those coupler moieties known in the art toform a colored reaction product with an oxidized color developer. Q andQ' represent the groups attached to the coupling carbon C, necessary tocomplete the dye-forming coupler moiety. Q and Q' may be independent ofeach other or together may represent a ring forming system to completethe dye-forming coupler moiety.

Examples of coupler moieties that may be used for yellow dye-formingcompounds are those derived from acylacetanilides such asbenzoylacetanilides and particularly pivaloylacetanilides and variationsof pivaloylacetanilides. Coupler moieties that may be used for magentadye-forming compounds are those derived from pyrazolotriazoles,indazolones, pyrazolobenzimidazoles, and particularly, pyrazolones suchas 1-aryl-5-pyrazolones. Coupler moieties that may be used for cyandye-forming compounds are those derived from substituted phenols orsubstituted naphthols, particularly 2-carbonamidophenols and1-hydroxy-2-naphthamides. As noted above, the formation of image dyes bythe reaction between a color-forming coupler and the oxidation productof a color developer in color photographic processes is well known, anda review of these color-forming reactions and color couplers usefultherein is found in James, T. H., The Theory of the PhotographicProcess, fourth ed., MacMillan Publishing Co., Inc., New York, 1977, pp.335-362.

The color developer moiety of the present invention may be any of thearomatic amino color developer moieties known or used in thephotographic art to form a colored reaction product with a dye-formingcoupler. The preferred color developer moieties are thep-phenylenediamines represented by the structural formula ##STR10##wherein R₁ and R₂ are each selected from hydrogen, alkyl, cycloalkyl,aryl, alkaryl, aralkyl, hydroxy substituted alkyl, sulfonamidosubstituted alkyl and alkoxy alkyl and is preferably lower alkylcontaining 1 to 6 carbon atoms; and X is hydrogen, alkyl, or substitutedalkyl, e.g., hydroxy or amino substituted alkyl, aryl, alkaryl, aralkyl,sulfo, carboxy, sulfonamido, and hydroxy. Particularly preferred colordeveloper moieties are the N,N-dialkyl-p-phenylenediamines, especiallythe N,N-diethyl-p-phenylenediamines. Other useful color developermoieties include p-aminophenols and certain amino-substitutedheterocyclic compounds, e.g., aminopyrazoline and aminohydroxypyrazoles.A review of color developers useful in color-forming reactions can befound in the above referenced James, T. H., The Theory of thePhotographic Process, fourth ed., MacMillan publishing Co., Inc., NewYork, 1977, pp. 335-362.

As stated above, the leuco dye of the present invention must contain aGroup 5A/Group 6A atom within either the dye-forming coupler moiety orthe color developer moiety to which the thermally migratable acyl groupis capable of migrating. Thermally induced acyl migrations to and fromcombinations of Group 5A and Group 6A atoms are known to occur. Examplesof such migrations have been described, for example, by Vyas, K.,Manohar, H., and Venkatesan, K., J. Phys. Chem., 94(15), 6069-73 (1990)whereby salicylamides undergo a thermally induced [1,5] O to N acylmigration. A [1,5] migration is one in which the migrating groupmigrates to an atom located 5 atoms away from the atom where themigrating group was originally positioned. Stoss, P. and Satzinger, G.,Chem. Ber., 111(4), 1453-63 (1978) describe a thermally induced [1,5] Nto N acyl migration in dibenzothiazepine imide oxides. Other thermallyinduced acyl migrations among and between Group 5A and 6A atoms havebeen described including [1,2], [1,3] and [1,5] migrations.

Illustrative coupler moieties containing a Group 5A/Group 6A atom whichmay be used for the yellow dye-forming compounds of the presentinvention include those couplers having the structural formula ##STR11##wherein R¹ is selected from (CH₃)₃ C--, CH₃ OCH₂ (CH₃)₂ C--, C₆ H₅O(CH₃)₂ C-- and phenyl, unsubstituted or substituted with one or moregroups selected from alkyl, alkoxy, nitro, halo such as chloro, andcarbonamido; R² is phenyl, unsubstituted or substituted with one or moregroups selected from alkyl, alkoxy, nitro, halo such as chloro, andcarbonamido, said phenyl group R² being the same or different from saidphenyl group R¹ ; and N is the requisite Group 5A atom.

Illustrative dye-forming coupler moieties containing a Group 5A/Group 6Aatom which may be used for the magenta dye-forming compounds of thepresent invention include ##STR12## wherein W is selected frombenzimidazolyl and phenyl, unsubstituted or substituted with one or moregroups selected from alkyl, alkoxy, amino, amino substituted with phenylor substituted with one or two alkyl groups and halo such as chloro;and, Z represents an organic side chain containing a Group 5A/Group 6Aatom to which said tM can migrate. The preferred Group 5A/Group 6A atomis nitrogen and representative Z groups containing nitrogen include--(CH₂)_(x) NHR³, amino, amino substituted with one phenyl or with onealkyl group, heterocyclic amino, carbonamido , sulfonamido, guanidino(N═C(NH₂)NHR), and ureido (NHCONHR) wherein x is 0, 1, 2, or 3 and R³ ishydrogen, alkyl or aryl.

Illustrative coupler moieties containing the requisite Group 5A/Group 6Aatom which may be used for the cyan dye-forming compounds arerepresented by ##STR13## wherein Z has the same meaning as above.

Illustrative color developer moieties containing a Group 5A/Group 6Aatom which may be used in the present invention include thoserepresented by the formula ##STR14## wherein R₁, R₂ and Z have the samemeaning as above and preferably, Z is positioned ortho to the nitrogenatom of the color developer moiety which is directly attached to thecoupler carbon.

Illustrative leuco dyes of the present invention and the dyes obtainedupon heating them are shown below: ##STR15##

In a preferred embodiment, the leuco dyes of the present invention andthe novel image dyes derived therefrom, upon heating, may be representedas in Scheme I: ##STR16## wherein R⁴ is a tertiary butyl (hereinaftert-butyl) group or other group having a quaternary carbon atom bonded tothe ketone carbonyl function, e.g., CH₃ OCH₂ C(CH₃)₂ --, C₆ H₅ OC(CH₃)₂--, 1-methylcyclohexyl, and is preferably t-butyl, i.e., (CH₃)₃ C--; R⁵is phenyl, unsubstituted or substituted with one or more groups selectedfrom alkyl, alkoxy, nitro, halo such as chloro, and carbonamido; E is athermally removable leaving group as defined above and is preferably aphenoxy group, unsubstituted or substituted with one or more groups, forexample, alkyl having 1 to 20 carbon atoms, alkoxy having 1 to 20 carbonatoms, and carboalkoxy having 1 to 20 carbon atoms; tM is a thermallymigratable acyl group as defined above, preferably --CO₂ CH₂ Ar, --CO₂R, and --GOR where Ar represents a substituted or unsubstituted phenylring; R is alkyl or aralkyl having 1 to 12 carbon atoms; and R⁶ and R⁷are selected from hydrogen and lower alkyl groups containing 1 to 6carbon atoms, particularly ethyl.

The leuco dyes of the present invention are synthesized by the oxidativecoupling of a color developer, e.g., a p-phenylenediamine substitutedwith a thermally migratable acyl group and color forming couplersubstituted in the coupling position with a thermally removable leavinggroup as described, for example, in the copending U.S. patentapplication (Ser. No. 277,014) of Roger Boggs, et al. The oxidizingagent may be any oxidizing agent conventionally employed, e.g.,potassium permanganate or potassium hexacyanoferrate(III) and ispreferably potassium hexacyanoferrate(III).

The dye-forming couplers substituted in the coupling position with aleaving group may be prepared by an analogous procedure to thatdescribed in U.S. Pat, No. 3,929,484 to Robert Ross issued Dec. 30,1975.

The dyes of this invention may be prepared by heating the leuco dyes ofthis invention under the appropriate conditions. Alternatively, the dyesmay be made by N-acylating the corresponding azomethine dye. TheN-acylation may be accomplished by any of the various methods known foracylation including reacting the dye with an acyl halide or acidanhydride under basic conditions. The azomethine dyes can be prepared byprocedures known in the art, e.g., those described in the aforementionedJames, T. H., The Theory of the Photographic Process, Fourth Ed.,MacMillan Publishing Co., Inc., New York, 1977, pp. 337-362.

The following examples are given to further illustrate the presentinvention and are not intended to limit the scope thereof.

EXAMPLE 1 Preparation ofN-[4,4-dimethyl-2-[N-[4-diethylaminophenyl]-N-acetylamino]-3-oxo-2-phenoxypentanoyl]-2,4-dichloroaniline,hereinafter "Leuco Dye A", having the structural formula ##STR17##

(1) Triethylamine (22.3 g, 30.6 mL, 0.22 mol) was added to a suspensionof N,N-diethyl-p-phenylenediamine hydrochloride (20 g, 0.1 mol) indichloromethane (150 mL) at room temperature. The mixture was cooled to0° C. and acetyl chloride (7.85 g, 0.1 mol) was added dropwise, withstirring. The mixture was allowed to warm to room temperature, and theprecipitate which had formed was removed by filtration. The solidresidue was washed with dichloromethane, and the combineddichloromethane solutions were concentrated. The dark oil produced wastriturated with hexanes (100 mL) to afford a light brown crystallinesolid, which was purified by recrystallization from 50% aqueous ethanolto afford the N-acetyl-4-diethylaminoaniline (16.5 g, 80% yield) as agray solid. The structure of this compound was confirmed by massspectroscopy and by ¹ H NMR spectroscopy.

(2) N-[4,4-dimethyl-3-oxo-2-phenoxypentanoyl]-2,4-dichloroaniline wasprepared in three steps using a method analogous to that described inthe aforementioned U.S. Pat. No. 3,929,484.

Step (i). Methyl 4,4-dimethyl-3-oxopentanoate (100 g, 0.632 mol) and2,4-dichloroaniline (100 g, 0.617 mol) were heated at reflux for 6 hoursin xylenes (reagent grade, dried over 4A molecular sieves, 150 mL) in aflask equipped with mechanical stirrer and distillation apparatus. 100mL of distillate was collected during this period. The reaction mixturewas then cooled to room temperature and petroleum ether (500 mL) wasadded with stirring. The mixture was cooled further with an ice bath,and the crystals which had formed were collected by filtration andwashed with petroleum ether. A second crop was collected afterconcentration of the filtrate. The combined materials were dried underhigh vacuum to give N-[4,4,-dimethyl-3-oxopentanoyl]-2,4-dichloroaniline(152.8 g, 84% yield) as a white solid which melted at 77°-78° C. Thestructure of this compound was confirmed by mass spectroscopy and by ¹ Hand ¹³ C NMR spectroscopy.

Step (ii). Sulfuryl chloride (54.1 g, 0.4 mol) was added dropwise withvigorous stirring over a 60 minute period to a solution ofN-[4,4-dimethyl-3-oxopentanoyl]-2,4-dichloroaniline, prepared as above(109 g, 0.378 mol) in dichloromethane (600 mL) at -10° C. The reactionmixture was maintained at this temperature for 3 hours after theaddition had been completed. After this time, the mixture was allowed towarm to 0° C. and ice (approximately 50 mL) was added, with stirring.The organic layer was separated and washed with 50% saturated aqueoussodium chloride solution (2×500 mL), then dried over magnesium sulfate.Evaporation of the solvent afforded a brown oil which was stirred withethanol (150 mL) to induce crystallization of the product. The resultantmixture was stored at 0° C. overnight. The solid product was removed byfiltration, washed with cold ethanol and dried to affordN-[2-chloro-4,4,-dimethyl-3-oxopentanoyl]-2,4-dichloroaniline (104.2 g,85.4% yield) as a white crystalline solid which melted at 64.5°-65.5° C.The structure of this compound was confirmed by mass spectroscopy and by¹ H and ¹³ C NMR spectroscopy.

Step (iii). Triethylamine (57.35 g, 566 mmol) was added to a stirredsolution ofN-[2-chloro-4,4,-dimethyl-3-oxopentanoyl]-2,4-dichloroaniline preparedas above (155.9 g, 483 mmol) and phenol (49.9 g, 530 mmol) in dryacetonitrile (1.3 L) under argon. After the addition had been completed,the mixture was heated at reflux for 7 hours. The reaction mixture wasthen cooled and the solvent was removed under reduced pressure. Theresidue was dissolved in methanol (300 mL) and allowed to stir at roomtemperature until crystallization began. After 2 hours, the mixture wascooled in an ice bath for a further 1 hour. The product was removed byfiltration, washed with cold methanol, and dried to affordN-[4,4-dimethyl-3-oxo-2-phenoxypentanoyl]-2,4-dichloroaniline (99 g, 54%yield) as a white crystalline solid which melted at 112°-113.5° C. Thestructure of this compound was confirmed by mass spectroscopy and by ¹ Hand ¹³ C NMR spectroscopy.

(3) A 5% aqueous solution of sodium carbonate (89 mL) was added to asolution ofN-[4,4-dimethyl-3-oxo-2-phenoxypentanoyl]-2,4-dichloroaniline (1.92 g,5.05 mmol) and N-acetyl-4-diethylaminoaniline (1.07 g, 5.19 mmol) inethyl acetate (35 mL). To the vigorously stirred mixture so formed wasthen added, in one portion, a solution of potassiumhexacyanoferrate(III) (3.33 g, 10 mmol) in water (35 mL). The resultantmixture was stirred for 6 hours then allowed to stand for a further 18hours. The ethyl acetate layer was then separated and the aqueous layerwas washed with more ethyl acetate (2×10 mL). The combined organiclayers were washed with a saturated sodium chloride solution and driedover sodium sulfate. After removal of the solvent, the crude product waspurified by flash chromatography on silica gel with 1:1-1:0dichloromethane/hexanes as eluant, followed by further flashchromatography on silica gel with 1:9 ethyl acetate/hexanes as eluant.Recrystallization from hexanes gave Leuco Dye A (253 mg, 9% yield) as awhite solid which decomposed at 126°-128° C. The structure of thiscompound was confirmed by mass spectroscopy and by ¹ H and ¹³ C NMRspectroscopy.

EXAMPLE 2 Preparation ofN-[4,4-dimethyl-2-[N-[4-diethylaminophenyl]-N-[ethoxycarbonyl]amino]-3-oxo-2-phenoxypentanoyl]-2,4-dichloroaniline,hereinafter "Leuco Dye B", having the structural formula ##STR18##

(1) Ethyl chloroformate 5.0 mL, 52 mmol) was added to a stirred mixtureof N,N-diethyl-p-phenylenediamine hydrochloride (10 g, 50 mmol) andsodium hydrogen carbonate (23.0 g) in dichloromethane (120 mL) at roomtemperature under nitrogen. After four days at room temperature, theprecipitate which had formed was removed by filtration through a pad ofcelite. The solid residue was washed with dichloromethane and thecombined dichloromethane extracts were concentrated to produce a blackoil, which was redissolved in dichloromethane (110 mL) and stirred withsilica (100 cc) for 30 minutes. The silica was then removed byfiltration through celite. The residue was washed with additionaldichloromethane (3×30 mL) and the combined dichloromethane extracts wereconcentrated to give N-ethoxycarbonyl-4-diethylaminoaniline (8.55 g, 73%yield) as a pale gray solid which melted at 31°-33° C. The structure ofthis compound was confirmed by mass spectroscopy and by ¹ H and ¹³ C NMRspectroscopy.

(2) A 5% aqueous solution of sodium carbonate (89 mL) was added to asolution ofN-[4,4-dimethyl-3-oxo-2-phenoxypentanoyl]-2,4-dichloroaniline, preparedin Example 1 above, (1.9 g, 5 mmol) andN-ethoxycarbonyl-4-diethylaminoaniline (1.73 g of 70% pure material,5.12 mmol) in ethyl acetate (35 mL). To the vigorously stirred mixtureso formed was then added, in one portion, a solution of potassiumhexacyanoferrate(III) (3.5 g, 10.6 mmol) in water (35 mL). The resultantmixture was stirred for 4 hours, after which the ethyl acetate layer wasseparated and the aqueous layer was washed with additional ethylacetate. The combined organic layers were washed with a saturated sodiumchloride solution and dried over sodium sulfate. After removal of thesolvent, the crude product was purified by repeated flash chromatographyon silica gel with 1:10 ethyl acetate/hexanes as eluant to give LeucoDye B (1.0 g, 33% yield) as a white foam. The structure of this compoundwas confirmed by mass spectroscopy and by ¹ H and ¹³ C NMR spectroscopy.

EXAMPLE 3 Preparation ofN-[4,4-dimethyl-2-[N-[4-diethyl-aminophenyl]-N-[benzyloxycarbonyl]-amino]-3-oxo-2-phenoxypentanoyl]-2,4-dichloroaniline,hereinafter "Leuco Dye C", having the structural formula ##STR19##

(1) Benzyl chloroformate (15.7 mL, 0.11 mol) was added dropwise to astirred mixture of N,N-diethyl-p-phenylenediamine hydrochloride (20 g,0.1 mol) and sodium hydrogen carbonate (42.0 g) in dichloromethane atroom temperature under nitrogen. After 16 hours at room temperature, theprecipitate which had formed was removed by filtration. The solidresidue was washed with dichloromethane and the combined dichloromethaneextracts were concentrated to produce a brown oil. This material waspurified by preparative high pressure liquid chromatography (hereinafterHPLC) on silica gel eluting with 10% hexanes/dichloromethane followed bydichloromethane alone. The resultant oil was triturated with hexanes togive N-benzyloxycarbonyl-4-diethylaminoaniline (18.25 g, 61% yield) as acrystalline solid. The structure of this compound was confirmed by massspectroscopy and by ¹ H NMR spectroscopy.

(2) A 5% aqueous solution of sodium carbonate (300 mL) was added to asolution ofN-[4,4-dimethyl-3-oxo-2-phenoxypentanoyl]-2,4-dichloroaniline, preparedas in Example 1 above, (5.00 g, 13.15 mmol) andN-benzyloxycarbonyl-4-diethylaminoaniline (3.923 g, 13.15 mmol) in ethylacetate (100 mL). To the vigorously stirred mixture so formed was thenadded, in one portion, a solution of potassium hexacyanoferrate(III)(9.1 g, 27.6 mmol) in water (100 mL). The resultant mixture was stirredfor 1.5 hours, after which the ethyl acetate layer was separated and theaqueous layer was washed with more ethyl acetate (50 mL). The combinedorganic layers were washed with a 50% saturated sodium chloride solutionand dried over sodium sulfate. After removal of the solvent, the crudeproduct was purified by HPLC on silica gel with 1:1dichloromethane/hexanes as eluant. Recrystallization from hexanes gaveLeuco Dye C (2.2 g, 25% yield). The structure of this compound wasconfirmed by mass spectroscopy and by ¹ H and ¹³ C NMR spectroscopy.

EXAMPLE 4 Preparation ofN-[2-[4-carbomethoxyphenoxy]-4,4-dimethyl-2-[N-[4-diethylaminophenyl]-N-acetylamino]-3-oxopentanoyl]-2,4-dichloroaniline,hereinafter "Leuco Dye D", having the structural formula ##STR20##

(1) HCl gas was bubbled into a suspension ofN-[2-[4-carboxyphenoxy]-4,4-dimethyl-3-oxopentanoyl]-2,4-dichloroaniline(12.0 g, 28.3 mmol, available from Eastman Kodak, Rochester, N.Y.) inabsolute methanol (175 mL) for 30 minutes. The mixture was heated toreflux for 2 hrs during which complete solution was obtained. Uponcooling to room temperature, white crystals of the product began toform. The reaction mixture was cooled in an ice bath, and the solidswere collected by filtration, washed with fresh methanol and air-driedto provideN-[2-[4-carbomethoxyphenoxy]-4,4-dimethyl-3-oxopentanoyl]-2,4-dichloroaniline(8.4 g, 68% yield). The structure of this compound was confirmed by massspectroscopy and by ¹ H and ¹³ C NMR spectroscopy.

(2) A 5% aqueous solution of sodium carbonate (200 mL) was added to asolution ofN-[2-[4-carbomethoxy-phenoxy]-4,4-dimethyl-3-oxopentanoyl]-2,4-dichloroaniline (2.192 g, 5.0 mmol) andN-acetyl-4-diethylaminoaniline, prepared as in Example 1 above, (1.032g,5.0 mmol) in ethyl acetate (50 mL). To the mixture so formed was thenadded in one portion, with vigorous stirring, a solution of potassiumhexacyanoferrate(III) (3.457 g, 10 mmol) in water (50 mL). The resultantmixture was stirred for one hour, after which the ethyl acetate layerwas separated, washed with a 50% saturated sodium chloride solution anddried over sodium sulfate. After removal of the solvent, the crudeproduct was purified by silica gel chromatography followed byrecrystallization from hexanes to yield Leuco Dye D (550 mg, 17% yield).The structure of this compound was confirmed by mass spectroscopy and by¹ H NMR spectroscopy.

EXAMPLE 5 Preparation ofN-acetyl--N-[4,4-dimethyl-2-[4-diethylaminophenyl]-imino-3-oxopentanoyl]-2,4-dichloroaniline,hereinafter "Dye A", having the structural formula ##STR21##

(1)N-[4,4-dimethyl-2-[4-diethylaminophenyl]imino-3-oxopentanoyl]-2,4-dichloroanilinewas prepared using a method analogous to that described in J. Korinek,J. Poskocil and J. Arient, Collection Czechoslov. Chem. Commun., 1979,44, 1460. A solution ofN-[2-[4-carboxyphenoxy]-4,4-dimethyl-3-oxopentanoyl]-2,4-dichloroaniline(10.0 g, 23.6 mmol, available from Eastman Kodak, Rochester, N.Y.) in amixture of 1M aqueous sodium hydroxide solution (100 mL) and ethanol (50mL), together with a solution of N,N-diethyl-p-phenylenediaminehydrochloride (5.20 g, 26 mmol) in ethanol (50 mL), was mixed all atonce with a solution of potassium persulfate (7.0 g, 26 mmol) in water(250 mL) containing ethanol (50 mL). The yellow reaction mixture wasstirred for one hour, and then extracted with dichloromethane (200 mL).The organic layer was separated and washed with a 50% saturated aqueoussodium chloride solution (200 mL) and dried over sodium sulfate.Evaporation of the solvent gave 11.4 g of crude material, which wastaken up in dichloromethane (200 mL) and slurried with silica gel (40g). After filtering, the solvent was removed to afford an oil (12 g)which was crystallized from hexanes (80 mL) to yieldN-[4,4-dimethyl-2-[4-diethylaminophenyl]imino-3-oxopentanoyl]-2,4-dichloroanilineas an orange solid (7.1 g, 68% yield). The compound exhibited a maximumabsorption in the visible region of the electromagnetic spectrum at 440nm, ε=20,600. The structure of this compound was further confirmed bymass spectroscopy and by ¹ H and ¹³ C NMR spectroscopy.

(2) Acetic anhydride (0.13 mL, 138 mmol) was added to a solution ofN-[4,4-dimethyl-2-[4-diethylaminophenyl]imino-3-oxopentanoyl]-2,4-dichloroaniline(445 mg, 1 mmol) and 4-dimethylaminopyridine (DMAP, 123 mg, 1.0 mmol) indichloromethane (5 mL) at room temperature and the resultant solutionwas allowed to stand for 3 days. The solvent was then removed, and thecrude product was purified by repeated flash chromatography on silicagel with 3% ethyl acetate/hexanes as eluant to give Dye A (243 mg, 50%yield) as an orange foam. The dye had a principal absorption in thevisible region of the electromagnetic spectrum at 438 nm, ε=15,300. Thestructure of this compound Was confirmed by mass spectroscopy and by ¹ H¹³ C NMR spectroscopy.

EXAMPLE 6

Preparation ofN-ethoxycarbonyl-N-[4,4-dimethyl-2-[4-diethylaminophenyl]imino-3-oxopentanoyl]-2,4-dichloroaniline,hereinafter "Dye B", having the structural formula ##STR22##

Ethyl chloroformate (0.15 mL, 1.57 mmol) was added to a solution ofN-[4,4-dimethyl-2-[4-diethylaminophenyl]-imino-3-oxopentanoyl]-2,4-dichloroaniline,prepared as in Example 5 above (440 mg, 0.98 mmol), triethylamine (0.15mL, 1.08 mmol) and DMAP (123 mg, 1 mmol) in dry dichloromethane (5 mL)and the resultant solution was stirred for 11 days under nitrogen. Thesolvent was then removed, and the crude product was purified by flashchromatography on silica gel with 3% ethyl acetate/hexanes as eluant togive slightly impure Dye B (530 mg). Further purification was effectedby a second chromatographic separation on silica gel using the sameeluant as before. The resultant Dye B, a red oil, had a principalabsorption in the visible region of the electromagnetic spectrum at 438nm, ε=17,000. The structure of this compound was confirmed by massspectroscopy and by 1H and ¹³ C NMR spectroscopy.

EXAMPLE 7 Preparation ofN-benzyloxycarbonyl-N-[4,4-dimethyl-2-[4-diethylaminophenyl]imino-3-oxopentanoyl]-2,4-dichloroaniline,hereinafter "Dye C", having the structural formula ##STR23##

Benzyl chloroformate (0.16 mL, 1.12 mmol) was added to a solution ofN-[4,4-dimethyl-2-[4-diethylaminophenyl]imino-3-oxopentanoyl]-2,4-dichloroaniline,prepared as in Example 5 above (478 mg, 1.07 mmol), triethylamine (0.15mL, 1.08 mmol) and DMAP (135 mg, 1.1 mmol) in dry dichloromethane (5 mL)and the resultant solution was stirred for 3 hours under nitrogen.Additional benzyl chloroformate (0.2 mL, 1.4 mmol) was then added,followed one hour later by more benzyl chloroformate (0.2 mL, 1.4 mmol)and additional triethylamine (0.15 mL, 1.08 mmol). The resultantsolution was stirred under nitrogen for 7 days, after which the solventwas removed. The crude product was purified by flash chromatography onsilica gel with 3% ethyl acetate/hexanes as eluant to give slightlyimpure Dye C. Further purification was effected by a secondchromatographic separation on silica gel in the same manner as above.Dye C (240 mg, 39% yield) was obtained as an orange foam which had aprincipal absorption in the visible region of the electromagneticspectrum at 438 nm, ε=15,900. The structure of this compound wasconfirmed by mass spectroscopy and by ¹ H and ¹³ C NMR spectroscopy.

EXAMPLE 8

As proof that the synthetic Dyes A, B and C of Examples 5, 6 and 7,respectively, are identical to the products of the laser-induced heatingof Leuco Dyes A, B and C of Examples 1, 2 and 3, carried out inexperiment 10 below, the following HPLC analysis was conducted.

Dichloromethane extracts of films A, B and C (containing Leuco Dyes A, Band C, prepared as described in Example 10 below), which had beenexposed as described in Example 10 at a scan speed of 0.4 m/s to givethe densities shown in Table 2, were analyzed by HPLC using aHewlett-Packard 1090M Liquid Chromatograph with diode array detector.The column employed was a reversed-phase Type AQ-303, having 5 micronparticle size and 120 A pore size, available from YMC Inc., 51 GibraltarDrive, Morris Plains, N.J. It was eluted with acetonitrile/watermixtures, in such a way that each component of the extracted film waswell resolved as a single peak. A single yellow product was observed foreach extracted film. The retention times and UV/VIS spectra of theyellow products from films A, B and C were compared with those ofsamples of the corresponding Dyes A, B and C prepared as described inExamples 5, 6 and 7. In each case they were found to be identical withinthe limits of experimental error.

EXAMPLE 9

In order to determine conclusively that acyl migration had occurred tothe nitrogen atom of the amide functional group in Leuco Dyes A, B and Cof Examples 1, 2 and 3, definitive confirmation of the structures ofDyes A, B and C was obtained by comparison of their respective ¹³ C NMRchemical shifts with those of an analogous Dye whose structure wasconclusively determined by crystal x-ray analysis.

The X-ray crystallographic analysis was performed on Dye X,N-tert-butoxycarbonyl-N-[4,4-dimethyl-2-[4-diethylaminophenyl]imino-3-oxopentanoyl]-2,4-dichloroaniline,having the structural formula shown below: ##STR24##

Dye X was prepared in a manner similar to that employed in the synthesisof Dyes A, B and C (Examples 5, 6 and 7) as follows:

A solution of di-tert-butyl dicarbonate (240 mg, 1.1 mmol) indichloromethane (5 mL) was added to a solution ofN-[4,4-dimethyl-2-[4-diethylaminophenyl]imino-3-oxopentanoyl]-2,4-dichloroaniline(448.4 mg, 1.0 mmol, prepared as described in Example 5 above) and N,N-dimethylaminopyridine (134.4 mg, 1.1 mmol) in dichloromethane (5 mL).The mixture was stirred overnight, after which work-up afforded Dye X asa yellow solid which could be crystallized from methanol. The structureof this compound was confirmed by mass spectroscopy and by ¹ H and ¹³ CNMR spectroscopy.

Single crystal X-ray analysis was performed on a crystal of Dye X grownfrom methanol. A light yellow needle of approximate dimensions0.08×0.13×0.50 mm was selected for data collection on a Nicolet P1-bardiffractometer with graphite monochromatized MoKα radiation. Celldimensions were determined from 12 reflections in the interval17°<2θ<22°. Intensities of 1493 reflections were collected using the ωscan mode with variable scan speeds between 2.93-6.51 degrees/min. Fourstandard reflections were measured every 100 reflections with nosignificant loss of intensity. The structure was solved by directmethods with SHELX86 and refined by full-matrix least squares withanisotropic displacement parameters for Cl atoms, isotropic displacementparameters for C, O, N and fixed H atoms (R =0.102, Rw=0.092). Themaximum height in the final difference map was 0.50e⁻ /Å³.

Cell parameters and non-hydrogen atom fractional crystal coordinates forDye X are shown in Table 1 below.

                  TABLE 1                                                         ______________________________________                                        Crystal system: monoclinic                                                                        Space group: C2/c                                         a = 33.217(17)Å β = 100.16(5)°                                b = 6.419(3)Å   V = 5896(10)Å.sup.3                                   c = 28.094(14)Å ρcalc = 1.236 g/cm.sup.3                              Atom     x            y         z                                             C11      0.594389     -0.426502 0.946245                                      C12      0.460397     -0.845253 0.957516                                      O1       0.664952     -0.349627 0.869088                                      O2       0.584603     -0.605529 0.815594                                      O3       0.641386     -0.893272 0.953575                                      O4       0.684307     -0.799986 0.904381                                      N1       0.664875     -0.723292 0.781875                                      N2       0.582867     -1.354591 0.667544                                      N3       0.615062     -0.761964 0.882627                                      C1       0.723526     -0.408926 0.831967                                      C2       0.720803     -0.334767 0.77915                                       C3       0.748749     -0.603423 0.840285                                      C4       0.743176     -0.239659 0.866488                                      C5       0.680327     -0.455419 0.84147                                       C6       0.655865     -0.632265 0.818652                                      C7       0.643272     -0.885957 0.757534                                      C8       0.646375     -0.906201 0.707687                                      C9       0.627441     -1.056391 0.679265                                      C10      0.602617     -1.202442 0.695411                                      C11      0.600162     -1.200644 0.745321                                      C12      0.619263     -1.040014 0.773586                                      C13      0.561141     -1.530194 0.686392                                      C14      0.519093     -1.468329 0.686471                                      C15      0.585257     -1.370713 0.614927                                      C16      0.620586     -1.488475 0.606856                                      C17      0.616261     -0.663534 0.838942                                      C18      0.650219     -0.823448 0.912817                                      C19      0.673023     -0.934957 0.996559                                      C20      0.700945     -1.098221 0.984044                                      C21      0.694797     -0.739636 1.012443                                      C22      0.648028     -1.011912 1.034476                                      C23      0.574795     -0.790952 0.897252                                      C24      0.552982     -0.965122 0.880629                                      C25      0.515632     -0.964017 0.903333                                      C26      0.505503     -0.815532 0.933338                                      C27      0.528344     -0.644468 0.947779                                      C28      0.563836     -0.644293 0.926047                                      ______________________________________                                    

The structural analogy between Dyes A, B and C and Dye X was confirmedby ¹³ C NMR spectroscopy. Spectra were measured at 75 MHz indeuterochloroform solution. The chemical shifts of the amide carbonylcarbon atoms of the four dyes were, respectively, 171.4, 169 3, 169.2and 169.1 ppm. The difference in chemical shift for Dye A is due to thedifference of the migrating group as compared to the migrating group inthe other three dyes. The full ¹³ C NMR spectra for the dyes are listedbelow with chemical shifts in ppm:

Dye A: 205.3, 171.4, 169.1, 150.1, 148.0, 136.2, 134.0, 131.9, 130.3,129.0, 125.6, 123.2, 122.4, 111.6, 44.6, 44.4, 27.2, 24.3, 12.6 ppm.

Dye B: 205 3, 169.3, 152.5, 149.5, 148.3, 135.2, 133.5, 132.9, 131.6,129.7, 128.2, 126.1, 123.6, 111.5, 63.9, 44.6, 44.4, 27.5, 14.1, 12.6ppm.

Dye C: 205.2, 169.2, 152.6, 149.4, 148.3, 135.3, 134.5, 133.6, 132.7,131.5, 129.7, 128.5, 128.3, 127.8, 126.0, 123.2, 111.5, 69.1, 44.6,44.4, 29.7, 27.4, 12.6 ppm.

Dye X: 205.3, 169.1, 151.3, 150.0, 148.1, 134.8, 133.8, 133.4, 133.2,131.5, 129.6, 128.1, 125.9, 111.5, 84.7, 44.6, 44.4, 27.6, 27.5, 12.6ppm.

In producing images according to the present invention, the way in whichthe heat is applied or induced imagewise may be realized in a variety ofways, for example, by direct application of heat using a thermalprinting head or thermal recording pen or by conduction from heatedimage-markings of an original using conventional thermographic copyingtechniques. Preferably, selective heating is produced in theimage-forming layers by the conversion of electromagnetic radiation intoheat and preferably, the light source is a laser emitting source such asa gas laser or a semiconductor laser diode. The use of a laser source isnot only well suited for recording in a scanning mode but by utilizing ahighly concentrated light source, photoenergy can be concentrated in asmall area so that it is possible to record at high speed and highresolution. Also, it is a convenient way to record data as a heatpattern in response to transmitted signals such as digitized informationand a convenient way of preparing multicolor images by employing aplurality of laser sources that emit light of different wavelengths.

In the latter embodiment an infrared absorbing substance is employed forconverting infrared radiation into heat which is transferred to theheat-sensitive leuco dye to initiate the departure of the thermallyremovable leaving group and the migration of the thermally migratableacyl group to form the dye chromophore. Obviously, the infrared absorbershould be in a heat-conductive relationship with the heat-sensitivecompound, for example, in the same layer as the heat-sensitive compoundor in an adjacent layer. Although an inorganic compound may be employed,the infrared absorber preferably is an organic compound, such as acyanine, merocyanine, squarylium or thiopyrylium dye. Preferably theinfrared absorber is substantially non-absorbing in the visible regionof the electromagnetic spectrum so that it will not add any substantialamount of color to the Dmin areas, i.e., the highlight areas of theimage.

In the production of multicolor images, infrared absorbers arepreferably selected that absorb radiation at different predeterminedwavelengths above 700 nm. Each imaging layer can then be exposedindependently of the others by using an appropriate infrared absorber.As an illustration, the layers of heat-sensitive compound for formingyellow, magenta, and cyan may have infrared absorbers associatedtherewith that absorb radiation at 780 nm, 850 nm and 900 nm,respectively, and may be addressed by laser sources, for example,infrared laser diodes emitting light at these respective wavelengths sothat the three color-forming layers can be exposed independently of oneanother. While each layer may be exposed in a separate scan, it isusually preferred to expose all of the imaging layers simultaneously ina single scan using multiple laser sources of the appropriatewavelengths. Rather than using superimposed imaging layers, theheat-sensitive leuco dyes and associated infrared absorbers may bearranged in an array of side-by-side dots or stripes in a singlerecording layer.

In a further embodiment, multicolor images may be produced using thesame infrared absorbing compound in association with each of two or moresuperimposed imaging layers and exposing each imaging layer bycontrolling the depth of focussing of the laser source. In thisembodiment, the concentration of infrared absorber is adjusted so thateach of the infrared absorbing layers absorb approximately the sameamount of laser energy. It will be appreciated that controlling thefocussing depth to address each layer separately may be carried out incombination with the previous embodiment of using infrared absorbersthat selectively absorb at different wavelengths in which instance theconcentration of infrared absorber would have to be adjusted for thelaser energy since the first infrared dye would not absorb anysubstantial amount of radiation at the absorption peaks of the secondand third dyes and so forth.

Where imagewise heating is induced by converting light to heat as in theembodiments described above, the heat-sensitive element may be heatedprior to, during or subsequent to imagewise heating. This may beachieved using a platen or heated drum or by employing an additionallaser source for heating the element while it is being exposedimagewise.

The heat-sensitive elements of the present invention comprise a supportcarrying at least one imaging layer of the heat-sensitive leuco dyes ofthis invention, and may contain additional layers, for example, asubbing layer to improve adhesion to the support, interlayers forthermally isolating the imaging layers from each other, infraredabsorbing layers as discussed above, anti-static layers, andanti-abrasive topcoat layer, and an ultraviolet screening layer havingan ultraviolet absorber therein or other auxiliary layers. For example,an electroconductive layer may be included and imagewise color formationeffected by heat energy generated in response to an electrical signal.

Heat-sensitive leuco dyes of this invention are selected which willgenerate dyes possessing the desired colors or combination of colors andthis includes ultraviolet and infrared absorbing dyes. For multicolorimages, the leuco dyes selected may comprise the additive primary colorsred, green, and blue, the subtractive primaries yellow, magenta, andcyan or other combinations of colors, which combinations mayadditionally include black. As noted previously, the leuco dyes aregenerally selected to give the subtractive colors cyan, magenta, andyellow commonly employed in photographic processes to provide fullnatural color. Also, a leuco dye that forms a black dye can be selectedfor providing a black image or a leuco dye that forms an infrared dyecan be selected for forming an image readable, for example, by an IRoptical character reader.

The support employed may be transparent or opaque and may be anymaterial that retains its dimensional stability at the temperature usedfor image formation. Suitable supports include paper, paper coated witha resin or pigment, such as calcium carbonate or calcined clay,synthetic papers or plastic films, such as polyethylene, polypropylene,polycarbonate, cellulose acetate, polyethylene terephthalate andpolystyrene.

Usually the layer of heat-sensitive leuco dye contains a binder and isformed by combining the heat-sensitive leuco dye and a binder in acommon solvent, applying a layer of the coating composition to thesupport, and then drying. Rather than a solution coating, the layer maybe applied as a dispersion or an emulsion. The coating composition alsomay contain dispersing agents, plasticisers, defoaming agents, coatingaids and materials such as waxes to prevent sticking where thermal pensare used to apply the imagewise pattern of heat. In forming the layer(s)containing the heat-sensitive leuco dyes and the interlayers or otherlayers, temperatures should be maintained below levels that willinitiate the removal of the leaving group and migration of the tM groupso that the heat-sensitive leuco dyes will not be prematurely colored.

Any of the binders commonly employed in heat-sensitive recordingelements may be employed provided that the binder selected is inert,i.e., does not have any adverse effect on the heat-sensitive leuco dyeincorporated therein. Also, the binder should be heat-stable at thetemperatures encountered during image formation and it should betransparent so that it does not interfere with viewing of the colorimage. Where electromagnetic radiation is employed to induce imagewiseheating, the binder should also transmit the light intended to initiateimage formation. Examples of binders that may be used include polyvinylalcohol, polyvinyl pyrrolidone, methyl cellulose, cellulose acetatebutyrate, copolymers of styrene and butadiene, copolymers of methyl andethyl acrylate, polyvinyl acetate, polymethyl methacrylate, polyvinylchloride, poly(ethyloxazoline), polyvinyl butyral and polycarbonate.

EXAMPLE 10

As an illustration of the utility of the compounds of this invention,coatings A, B and C were made using the aforementioned Leuco Dyes A, Band C of Examples 1, 2 and 3 in the following manner:

A solution of the leuco dye (40 mg) in dichloromethane (0.17 mL) wascombined with an infrared absorbing dye,(1,3-bis[2,6-di-t-butyl-4H-thiopyran-4-ylidenemethyl]-2,4-dihydroxy-dihydroxide-cyclobutenediylium bis-inner salt (see U.S. Pat. No. 4,508,811), 0.13 mL of a 1%solution in dichloromethane and a polymeric binder, polyvinyl butyral(Butvar B-79, supplied by Monsanto, St. Louis, Mo.) 0.4 mL of a 10%solution in dichloromethane. The resultant solution was coated onto atransparent polyethylene terephthalate base of 4 mil thickness(commercially available from ICI Americas, Inc., Wilmington, Del.) usinga #8 coating rod. The film so formed was laminated to a second sheet ofpolyethylene terephthalate of 1.4 mil thickness (ICI type 3121, suppliedby ICI Americas, Inc., Wilmington, Del.) at 180° F. and 60 psi.

The above prepared coatings A, B, and C were exposed to infraredradiation from a GaAlAs semiconductor diode laser emitting at 819 nm,which delivered 127 mW to the medium. The laser output was focussed to aspot of about 33×3 microns in size. The coatings were wrapped around adrum whose axis was perpendicular to the incident light. The 4 mil baseof the laminated structure was in contact with the drum, so thatexposure of the infrared absorbing layer occurred through the 1.4 milbase. Rotation of the drum about its axis and simultaneous translationin the direction of the axis caused the laser spot to write a helicalpattern on the medium. The pitch of the helix was 33 microns, chosen sothat none of the medium was left unexposed between adjacent turns. Inthis arrangement, the exposure received by the medium was inverselyproportional to the speed of rotation of the drum (here measured as alinear speed at the medium surface).

The transmission optical density (hereinafter ODtrans) measured in bluelight for each of the coatings is set forth in Table 2 as a function ofdrum speed. It should be noted that the initial transmission opticaldensity (hereinafter Dmin) measured in blue light before exposure forcoatings A, B and C was measured and was 0.05. Density measurements wereobtained using a Macbeth TD-504 densitometer, using the appropriatefilter. Each figure is the mean of 10 readings, 5 taken in differentparts of each of two coatings prepared from the same coating fluid. Acomparison coating X was made in the same manner as above, except thatthe leuco dye employed was a prior art, heat-sensitive leuco dyesubstituted with a t-Boc protecting group to stabilize the colorlessform. The leuco dye, hereinafter Leuco Dye X, has the structural formula##STR25## and is disclosed and claimed in the aforementioned copendingpatent application of Roger Boggs, et al, Ser. No. 07/277,014, nowabandoned and replaced by copending U.S. patent application Ser. Nos.07/729,420 (now U.S. Pat. No. 5,192,645) and 07/729,426, both filed onJul. 12, 1991). The ODtrans measured for coating X is also set forth inTable 2 as is the degree of bubble formation in imaged areas for each ofthe four coatings.

The degree of bubble formation is designated High (H), Medium (M) or Low(L) according the following definitions:

"High"--bubbles of at least 1 mm diameter;

"Medium"--smaller bubbles which lead to scattering of light andconsequently to areas of the film which appear dark in transmittedlight;

"Low"--image quality is not compromised by bubbles.

It will be appreciated that where bubble formation is High, the Dmaxvalues reported in Table 2 are approximate due to the scattering oflight by bubbles.

                  TABLE 2                                                         ______________________________________                                                    Coating:                                                          Drum speed  A       B         C     X                                         (m/s)       ODtrans (Bubble formation)                                        ______________________________________                                        0.3         1.23(L) 1.27(L)   1.20(L)                                                                             1.49(H)                                   0.4         1.44(L) 1.54(L)   1.36(L)                                                                             1.43(H)                                   0.5         1.38(L) 1.32(L)   1.30(L)                                                                             1.37(M)                                   0.6         1.30(L) 1.24(L)   1.12(L)                                                                             1.29(L)                                   0.75        0.95(L) 0.82(L)   0.90(L)                                                                             0.82(L)                                   1.0         0.39(L) 0.30(L)   0.52(L)                                                                             0.38(L)                                   ______________________________________                                    

From the ODtrans values reported in Table 2, it can be seen that coloris formed at the various scanning rates for each of the sample coatingscomprising a leuco dye of the present invention clearly demonstratingthe utility of the leuco dyes as thermal imaging materials. Table 2 alsoreveals that the images generated with the leuco dyes of this inventionare not compromised by bubble formation regardless of the exposure,whereas the image generated by the coating containing the prior artleuco dye is compromised by bubble formation at higher exposures. Sincethe conversion of the leuco dye of this invention into colored dyes doesnot involve fragmentation of a thermally removable group, imagingsystems employing these leuco dyes have the significant advantage ofdecreased bubble formation relative to imaging systems which utilizeprior art leuco dyes substituted with thermally removable groups whichfragment upon heating to generate gases.

It will be appreciated that the leuco dyes of the present invention andthe heat-sensitive elements prepared therefrom may be used in variousthermal recording systems including thermal printing, thermographiccopying and, particularly, high resolution images suitable for viewablecolor prints and transparencies, color images requiring magnificationsuch as microfilm, color filters for color displays and color sensors,optical disks and so forth. Depending upon the particular application,the heat-sensitive elements may contain thermal isolating layers,reflective, subcoat, topcoat or other layers, and the various layersincluding the imaging layer(s) together with any infrared absorbinglayer(s) may be arranged in the configuration as appropriate.

Since certain changes may be made in the herein described subject matterwithout departing from the scope of the invention herein involved, it isintended that all matter contained in the above description and examplesbe interpreted as illustrative and not in a limiting sense.

We claim:
 1. A heat-sensitive recording element which comprises asupport carrying at least one layer of a leuco dye represented by##STR26## wherein: E represents a thermally removable leaving group;tMrepresents a thermally migratable acyl group; Q, Q' and C taken togetherrepresent a dye-forming coupler moiety wherein C is the coupling carbonof said coupler moiety; and, (Y) taken together with N represents anaromatic amino color developer moiety, one of said Q, Q' and (Y)containing an atom selected from the atoms comprising Group 5A/Group 6Aof the Periodic Table, said groups E and tM maintaining said leuco dyein a substantially colorless form until the application of heat causessaid group E to be eliminated from said leuco dye and said group tM tomigrate from said N atom to said Group 5A/Group 6A atom thereby forminga dye represented by ##STR27## wherein said dotted lines indicate thatsaid tM group is bonded to said Group 5A/Group 6A atom in one of said Q,Q' and (Y).
 2. A heat-sensitive recording element as defined in claim 1wherein said Group 5A/Group 6A atom to which said tM migrates isselected from a nitrogen atom, oxygen atom and sulfur atom.
 3. Aheat-sensitive recording element as defined in claim 1 wherein saidGroup 5A/Group 6A atom to which said tM migrates is a nitrogen atom. 4.A heat-sensitive recording element as defined in claim 1 wherein saidcoupler moiety is a yellow dye-forming coupler moiety having thestructural formula ##STR28## wherein R¹ is selected from (CH₃)₃ C--, CH₃OCH₂ (CH₃)₂ C--, C₆ H₅ O(CH₃)₂ C-- and phenyl, unsubstituted orsubstituted with one or more groups selected from alkyl, alkoxy, nitro,halo, and carbonamido; R² is phenyl, unsubstituted or substituted withone or more groups selected from alkyl, alkoxy, nitro, halo, andcarbonamido, said phenyl group R² being the same or different from saidphenyl group R¹.
 5. A heat-sensitive recording element as defined inclaim 1 wherein said aromatic amino color developer is ap-phenylenediamine.
 6. A heat-sensitive recording element as defined inclaim 1 wherein said aromatic amino color developer is represented by##STR29## wherein Z represents an organic side chain containing a Group5A/Group 6A atom to which said tM can migrate and R₁ and R₂, the same ordifferent, are each selected from hydrogen and lower alkyl groupscontaining 1 to 6 carbon atoms.
 7. A heat-sensitive recording element asdefined in claim 1 wherein said tM is represented by the structuralformula ##STR30## wherein R is selected from alkyl, cycloalkyl, aryl,aralkyl, alkaryl, heterocyclic and, ##STR31## wherein R' and R", thesame or different, are selected from hydrogen, alkyl, cycloalkyl, aryl,aralkyl, alkaryl and heterocyclic.
 8. A heat-sensitive recording elementas defined in claim I wherein said E is a phenoxy group, unsubstitutedor substituted with one or more groups selected from alkyl, alkoxy,halo, carboxy, nitro, cyano, --SO₂ alkyl, --SO₂ phenyl, tosyl andN,N-(dialkyl)amino.
 9. A heat-sensitive recording element as defined inclaim 1 which comprises at least two layers, each layer containing aleuco dye and additionally containing a thermal isolating layer betweenadjacent layers of leuco dye.
 10. A heat-sensitive recording element asdefined in claim 9 wherein an infrared absorber is associated with eachsaid layer of leuco dye.
 11. A heat-sensitive recording element whichcomprises a support carrying at least one layer of a leuco dye havingthe structural formula ##STR32## wherein R⁴ is a t-butyl group or othergroup having a quaternary carbon atom bonded to the ketone carbonylfunction; R⁵ is phenyl, unsubstituted or substituted with one or moregroups selected from alkyl, alkoxy, nitro, halo, and carbonamido; E is athermally removable leaving group; tM is a thermally migratable acylgroup; and R⁶ and R⁷ are selected from hydrogen and lower alkyl groupscontaining 1 to 6 carbon atoms, said thermally migratable acyl group andsaid thermally removable leaving group maintaining said leuco dye in itscolorless form until the application of heat causes the elimination ofthe thermally removable leaving group and the migration of the thermallymigratable acyl group to the N containing H and R⁵ whereby said leucodye is converted into an image dye represented by the structural formula##STR33##
 12. A heat-sensitive recording element as defined in claim 11wherein R⁴ is a t-butyl group.
 13. A heat-sensitive recording element asdefined in claim 11 wherein R⁶ and R⁷ are each ethyl.
 14. Aheat-sensitive recording element as defined in claim 11 wherein E is asubstituted or unsubstituted phenoxy group.
 15. A heat-sensitiverecording element as defined in claim 11 wherein said R⁵ is a2,4-dichlorophenyl group.
 16. A heat-sensitive recording element asdefined in claim 11 wherein said leuco dye has the structural formula##STR34## which is converted into the image dye having the formula##STR35## wherein tM represents a thermally migratable acyl group.
 17. Aheat-sensitive recording element as defined in claim 16 wherein tM isselected from --COCH₃, --CO₂ CH₂ Ph and --CO₂ C₂ H₅.
 18. Aheat-sensitive recording element as defined in claim 11 which comprisesat least two layers containing a leuco dye, and additionally containinga thermal isolating layer between adjacent layers of leuco dye.
 19. Aheat-sensitive recording element as defined in claim 18 wherein aninfrared absorber is associated with each said layer of leuco dye.
 20. Amethod of thermal imaging which comprises heating imagewise aheat-sensitive element comprising a support carrying at least one layerof a leuco dye having the structural formula: ##STR36## wherein: Erepresents a thermally removable leaving group;tM represents a thermallymigratable acyl group; Q, Q' and C taken together represent adye-forming coupler moiety wherein C is the coupling carbon of saidcoupler moiety; and (Y) taken together with N represents an aromaticamino color developer moiety, one of said Q, Q' and (Y) containing anatom selected from the atoms comprising Group 5A/Group 6A of thePeriodic Table, said groups E and tM maintaining said leuco dye in asubstantially colorless form until the application of heat causes saidgroup E to be eliminated from said leuco dye and said group tM tomigrate from said N atom to said Group 5A/Group 6A atom thereby forminga dye represented by ##STR37## wherein said dotted lines indicate thatsaid tM group is bonded to said Group 5A/Group 6A atom in one of said Q,Q' and (Y).
 21. A method of thermal imaging as defined in claim 20wherein said Group 5A/Group 6A atom is selected from a nitrogen atom,oxygen atom and sulfur atom.
 22. A method of thermal imaging as definedin claim 20 wherein an infrared absorber is associated with each layerof leuco dye for absorbing radiation at wavelengths above 700 nm andtransferring said absorbed radiation as heat to said leuco dye, saidlayer being heated imagewise by imagewise exposure to infrared radiationat a wavelength strongly absorbed by said infrared absorber.
 23. Amethod of thermal imaging as defined in claim 20 wherein said leuco dyehas the structural formula ##STR38## wherein R⁴ is a t-butyl group orother group having a quaternary carbon atom bonded to the ketonecarbonyl function; R⁵ is phenyl, unsubstituted or substituted with oneor more groups selected from alkyl, alkoxy, nitro, halo such as chloro,and carbonamido; E is a thermally removable leaving group; tM is athermally migratable acyl group; and R⁶ and R⁷ are selected fromhydrogen and lower alkyl groups containing 1 to 6 carbon atoms, saidthermally migratable acyl group and said thermally removable leavinggroup maintaining said leuco dye in its colorless form until theapplication of heat causes the elimination of the thermally removableleaving group and the migration of the thermally migratable acyl groupto the N containing H and R⁵ whereby said leuco dye is converted into anovel image dye represented by the structural formula ##STR39##
 24. Amethod of thermal imaging as defined in claim 23 wherein said leuco dyehas the structural formula ##STR40## which is converted into the imagedye having the formula ##STR41## wherein tM represents a thermallymigratable acyl group.
 25. A method of thermal imaging as defined inclaim 24 wherein tM is selected from --COCH₃, --CO₂ CH₂ Ph and --CO₂ C₂H₅.